In the modern highway vehicles of the tractor and semi-trailer type, one or more air pressure reservoirs are employed on the trailer to supply air pressure to maintain spring-applied air-released parking brakes in the released condition. Control of the stored air is provided by a supply air conduit from the tractor which both maintains the trailer pressure reservoirs in the pressurized condition and also acts through an amplifying relay valve to connect the air pressure from at least one of the air pressure reservoirs to the parking brakes. When the supply conduit is vented, either by the vehicle operator operating a parking brake control in the cab, or by intentional or accidental disconnection of the air pressure lines between tractor and trailer, the amplifying relay valve vents the spring-set parking brakes on the trailer and thereby sets the parking brakes.
When a semi-trailer is parked in a parking area or when a vehicle malfunction has stranded the trailer on a road with the parking brakes set, it is frequently desirable to be able to release the trailer parking brakes using the air pressure stored on board. This would permit the trailer to be moved about a parking yard or moved out of a traffic area without the time and inconvenience to connect and pressurize the air pressure lines between the tractor and trailer.
In order to provide redundancy in actuation of the service brakes and the parking brakes, it is common practice to provide one or two service brake reservoirs for the control of service brakes and one emergency brake reservoir for the control of parking brakes.
Significant advances have been made in reducing the cost and weight of the braking systems by reducing the required number of air pressure reservoirs from four or three to two. In such systems, as typified by U.S. Pat. Nos. 3,992,064 and 4,042,281, two double-protected reservoirs are separately controllable to feed stored pressurized air to the vehicle service brakes and either or both pressure reservoirs are operative to supply air pressure for maintaining the parking brakes disengaged. Double protection of the reservoirs indicates that each reservoir is isolated from the other both at its input and its output and both are prevented from venting their stored air pressure back to the source.
The systems shown in U.S. Pat. Nos. 3,992,064 and 4,042,281 do not provide for manual control of the trailer parking brakes from the trailer. The problem arises because the control input to the relay valve is also connected through a pressure protection valve to the two reservoirs. In the event that one of the reservoirs is depleted, any air pressure attempted to be fed to the control input of the relay valve is bled off through the pressure protection valve into the depleted reservoir until the pressure tank having higher pressure is depleted until its air pressure is either equal to the depleted tank or is reduced to the pressure limit established by the pressure protection valve. Consequently, even though one of the two pressure tanks may contain more than enough air pressure to both operate the parking brakes and perform the control function, there is no way that this control can be performed.